In this paper, the performance limits, as given by the signal-to-noise ratio (s/n), are described for different speech-encoding schemes including adaptive quantization and (linear) adaptive prediction schemes. The comparison is made on the basis of computer simulations using 8-kHz-sampled speech signals of one speaker. Different bit rates (two bits per sample–five bits per sample) have been used. A three-bit-per-sample pcm scheme with a nonadaptive μ100 quantizer leads to an s/n value of approximately 9 dB. A maximum s/n value of approximately 25 dB has been reached using an encoding scheme including both adaptive quantization and adaptive prediction. Entropy coding of the quantizer output symbols leads to an additional gain in s/n of nearly 3 dB.

A comparative study of various quantization schemes for speech encoding

It is widely recognized that the electromagnetic spectrum is a beleaguered resource, and nowhere is this more evident than in the land-mobile radio service. The existing allocations of spectrum have long been oversubscribed in many major cities, and huge backlogs of potential users with no allocations have built up in many areas. Cellular land-mobile radio systems are widely recognized to provide a potential solution to this problem, and cellular systems using conventional analog modulation techniques have been proposed [l], [2]. This article discusses the authors' belief that spreadspectrum  can substantially improve the  spectral efficiency of a cellular land-mobile radio system, as well as making it more versatile and better adapted to the environment. It should be emphasized, however, that the theoretical work on which this belief is based is in a very preliminary stage and that much more study and simulation, as well as a field trial, are required before the validity of the claims made here can be established. Indeed, much of the work described here is controversial and, hence, a major function of this paper is to present these views to a wider audience and thereby encourage further discussion of them.

Cellular land-mobile radio: Why spread spectrum?

Various asynchronous delta modulation algorithms are considered. The statistical distribution of interbit intervals is analyzed and the signal-to-noise ratio behaviour is compared vis-a-vis linear and adaptive delta modulation algorithms. The problem of transmission of nonuniform samples through synchronous channels has also been treated. Quantization and runlength encoding characteristics of interbit intervals are studied. The optimal size and the statistical properties of buffer memory are determined and trade-offs between various system parameters such as runlength constraint, overflow and underflow probabilities and synchronization words are investigated.

Signal coding properties of asynchronous delta modulator

Current scientific efforts in the field of digital processing of speech are focused at the aims of improving the efficiency in the present state of the art, and of developing new digital speech communication systems. Therefore, thorough studies on the statistical characteristics of speech signals, speech coding, speech recognition, and speech synthesis are necessary. Recent results and actual trends are reviewed in this paper.

Recent Progress in Digital Processing of Speech

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Recent Progress in Digital Processing of Speech